Resonance Raman spectroscopy has been used as a probe of the structure of ligands at the active site of hemerythrin. Molecularly revealing insights have been obtained with oxyhemerythrin and with metazidohemerythrin. This spectroscopic technique has also facilitated a comparison of oxygen carrier within erythrocytes with that in solution. The electronic state of the bound O2 is the same in the natural environment as in the artificial one.